Electric impedance waveform generator



May 3, 1966 J. J. WARD ETAL 3,

ELECTRIC IMPEDANCE WAVEFORM GENERATOR Filed May 1, 1965 2 Sheets-Sheet 1PULSE GENERATOR J R/ W2 R2 "3 3 n n PL our Hal.

BY Mow e Wu Wei ATTORNEYS May 3, 1966 J. J. WARD ETAL ELECTRIC IMPEDANCEWAVEFORM GENERATOR Filed May 1, 1963 2 Sheets-Sheet 2 w W o r V C V a. rv 6. w. z N m v #0 a O v c E O++ N MM 7% DE MN m mm w m m Wm F o 1:: V AK P 6 w m r s z W o m8 S 3 W V V 3 L E 0 ATTORN EY-S United StatesPatent 3,249,879 ELECTRIC IMPEDANCE WAVEFORM GENERATOR John Joseph Ward,Maidenhead, and Brian Dennis Mc- Carthy, Catford, London, England,assignors to Specto Limited Filed May 1, 1963, Ser. No. 277,395

4 Claims. (Cl. 328-186) This invention relates to electric impedancenetworks, for the generation of voltages the amplitude waveform of whichis a predetermined function with respect to time.

One method of producing a complex voltage waveform, applicable to thosecases where it is not possible to use an oscillation generator of asimple recurrent type, is to establish a matrix of resistors or likeimpedance elements associated with a common output impedance. Theresistors are energized in rapid sequence from a potential source, as bygating circuits or the application of energizing pulses to resistors ofthe matrix, so as to produce a sequence of voltages across the commonoutput impedance. Each such voltage constitutes an elemental part of thedesired waveform, so that the sequence of voltages can be made toapproximate the desired voltage waveform, with an accuracy that dependsupon the number of discrete voltage steps that can be derived from theresistors of the matrix.

-The matrix may take various forms, including one in which a number ofresistors are connected all in series, and in series with a loadresistor, the desired instantaneous output voltage being obtained byapplying a supply voltage to an appropriate junction of two resistors;in a second form, the resistors are arranged individually in parallelforconnection in series with the common load resistor; in this latter casethe desired output is obtained by applying appropriate voltages insequence to the free ends of the parallel resistors.

The desired output can be obtained from the voltage which is developedacross the common load impedance, or it may be derived as the currentthrough the impedance. It can be shown that the two forms of arrangementare approximately equivalent, and that similar considerations apply tothem.

In the design of circuits of this kind difiiculty is encountered in theselection of an appropriate value for the common load resistor. In onerespect, the use of a very high value of load resistor is desirable; thehigher the value of the load resistance used, the closer becomes theapproximation to a linear relationship between the value of theindividual matrix resistors and the output voltage developed. Forexample, if a constant current device is connected in serieswith theseries or parallel resistors, and fed from a constant voltage source,the voltage across the load impedance will give a close approximation tothe desired proportionality between the value of the resistors of thematrix and the voltage produced across the load impedance. Such anarrangement is of very useful advantage when circuits are beingdesigned, as it tends to avoid the need for the use of resistors ofnon-standard values; this is especially the case where the parallelresistance circuit is used.

If this were the only consideration in the design of resistance matricesof this kind the use of a constant current device as the load impedancewould be satisfactory solution, but in most practical cases there areother considerations which greatly detract from the value of this formof circuit. For example, it is very desirable that it should be possibleto vary the amplitude of the resultant waveform without altering itsshape, that is, without altering the relative values of its components.For example, if in a simple case the waveform were a sine wave, it mightbe required pure sine wave.

to reduce its amplitude, while preserving its shape as a Where the loadimpedance has a very high value any such change of amplitude isdiflicult to effect by normal means, such as the use of a potentialdividing potentiometer, since the connection of the potentiometer to theload circuit, for'example, the constant current device will alter theeffective total load impedance, and thereby affect the proportionalitybetween the values of the matrix resistors and the voltages which theyproduce.

Another requirement that may arise, especially where the output waveformis used as a deflection controlling voltage for a cathode rayoscillograph, is that it should be possible to superimpose the waveformupon a direct voltage, and that it should be possible to vary thealternating and direct currents of this composite waveformindependently.

The present invention is concerned with a very simple circuit by meansof. which a satisfactory compromise can be obtained between the variousdesign requirements.

In the circuit of the present invention, the load resistance has afinite value, which means that some of the advantage of a very high loadimpedance is not obtained, but the'output voltage isapplied to a summingamplifier, which is charasterized by the fact that the input terminal ofthe amplifier is maintained at a substantially constant potential bymeans of voltage feedback from the output to the input of the amplifier.The degree of feedback is such that the forward gain of the amplifier isapproximately unity, or less. Specifically, a transistor amplifier isused for this purpose, and the voltage feedback path is established overa direct current path, such as a resistor, so that the circuit willoperate both as an alternating current and a direct current amplifier.By this means, it is possible to vary the gain of the amplifier, .andhence the amplitude of the alternating output signal, by means of thefeedback path, and the fact that the amplifier is direct coupled makesit a simple matter to establish any desired direct current component inthe composite alternating and direct current output.

Other features and advantages of the invention will appear from thefollowing description of one embodiment thereof, given by way ofexample, in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a first form of a function generator;

FIGURE 2 is a corresponding diagram of a further form of the functiongenerator;

FIGURE 3 is a partly block diagram of a load impedance for use in afunction generator;

FIGURES 4 and 5 are waveform diagrams of output voltages from -afunction generator;

FIGURE 6 is a circuit diagram of a typical resistor matrix and summingamplifier, and

FIGURE 7 is a waveform diagram.

FIGURE 1 shows a form of function generator comprising a matrix ofresistors R R R R The parallel combination of these resistors isconnected in series with a load resistor R To the resistors R R etc.,there are applied a succession of pulses of equal voltage amplitude andequal time duration, the pulses being sequential in time as indicateddiagrammatically by the waveform diagrams W1, W etc. The pulses areindicated as being generated by a pulse generator PG, supplied from asuitable source of potential, not shown. As a result of the pulsevoltage applied to each resistor of the matrix, there will appear acorresponding voltage across the load resistor R from the terminals ofwhich the output voltage of the generator is obtained. The amplitude ofthe voltage appearing across the load resistor will accordingly be afunction of the relative resistances A second form of the functiongenerator is indicated in FIGURE 2. This closely resembles that ofFIGURE 1, the major difierence being that the resistors of the matrixare in series instead of being in parallel, and that additionally theconnection to each resistor of the matrix is made through an isolatingdiode such as D D D Pulses are applied in sequence to the diodes D Detc., though not necessarily in that order, from pulse generator PG, asbefore.

In the practical case, the form of matrix shown in FIG- URE 2, usingserial resistors, is to be preferred to that of FIGURE 1 using theparallel resistors. The reason for this is that with the seriesarrangement the individual re sistors will represent not the absolutevalue of the desired waveform at any particular instant, but incrementalvoltages, representing the differences between voltages at consecutiveinstants of time. As a result of this, the values of the resistancesused in the matrix of FIGURE 2 will be much smaller than those used withthe matrix of FIG- URE 1, and can therefore be chosen with greaterabsolute accuracy. Apart from this, the circuits are generallyequivalent.

With a function generator of either of the forms the voltage across theload will be given by the relationship where V is the output voltagedeveloped across the load resistor R V is the input voltage applied tothe matrix, such as the pulse voltage, and R is the resistance of theindividual matrix resistor.

If the load resistance in this case is a constant current device, itwill be seen that the output voltage V is given by the relationshipwhere V is the voltage of the applied pulse or source, V is the voltagedeveloped across the constant current device and i is the current,determined by the constant current device. By its nature, the constantcurrent device will ensure that the voltage across it will be sensiblyconstant, so that in this case the output voltage will be directlyproportional to R As mentioned above, the use of a constant currentdevice for the load impedance is satisfactory, in that it .ensures thedesired direct proportionality between the value of the individual loadresistor R and the output voltage V produced, but it is not suitable inthose cases where it is desired to vary the amplitude of the output.

With the arrangement of the invention, a compromise value is adopted forthe load resistance R and the output voltage is applied to a summingamplifier, the characteristics of which make it suit-able forcontrolling the amplitude of the output Waveform and, if desired, theDC. level.

FIGURE 3 shows an arrangement of summing amplifier and load resistorwhich can be used with advantage with either of the resistor matricesdescribed. The load resistor R which is the load resistor of theimpedance matrix, feeds an amplifier AMP, from which an output issupplied to the output terminals OUT. A feedback re sistor R isconnected between the output and input terminals of the amplifier, so asto provide a form of voltage feedback. The effect of voltage feedback isto reduce the input impedance of the amplifier and if the degree offeedback is sufficiently large it can be arranged that the inputterminal of the amplifier will be maintained at substantially a constantpotential despite the applied input signal. In this condition, the gainof the amplifier, including the feedback, is substantially unity, orless. The gain can be controlled for design purposes, by giving to thefeedback resistor R a suitable value.

It will be seen that the feedback path is effective not only foralternating current but also for direct current, and when this is so itbecomes a possible to vary the DC. level of the output signal. A DC.signal can be used to supplement the waveform signal from the matrix,applied through a resistor R and applied to the input terminal of theamplifier. It is feasible to apply the desired D.C. potential to theoutput of the amplifier.

The feedfback circuit of amplifier AMP includes also a capacitor C Thiscapacitor will increase the feedback ratio with increasing'frequency andproduce a frequency selective response of the amplifier. This responsecan be used to smooth out those components of the input waveform whichare at high frequency, so that the system operates as a low pass filter,and smoothing the output waveform to that more nearly approaching thedesired waveform.

The pulses generated by the pulse generator PG will have finite rise andfall times, and in consequence the output voltage appearing across theload resistor may include spikes somewhat in the manner indicated inFIG- URE 4. These spikes are generally undesirable, and if this is so,the filter section shown in FIGURE 3 can be used in order substantiallyto reduce them. The etIect of using this further section of the networkfor the load resistor is to provide a circuit that has different timeconstants for falling and rising voltages. The section includes an inputresistor R feeding a buffer amplifier B. AMP and a diode D outputincludes a shunt capacitor C V As a result of these provisions, awaveform of the kind shown in FIGURE 4 will be modified somewhat asshown in FIGURE 5. It will be seen that the corners of the pulsesforming the composite wave are slightly rounded, and the spikes largelyeliminated.

In some circumstances, a similar removal of the spikes between theadjacent components of the composite waveform can be effected by anothermeans: by providing that the pulses which are applied to the successiveinputs of the resistor matrix overlap very slightly in time. An overlapof the order of 2% to 5% in a practical case, has been found suitable.This overlap in conjunction with the finite rise and fall times of apractical pulse, can be so arranged as substantially to eliminate thespikes, without the other circuit provisions.

FIGURE 6 shows a more detailed circuit arrangement of one specificresistor matrix, for producing a waveform such as that shown in FIGURE7b. It is assumed that the waveform can be sufiiciently preciselyrepresented from a total of 32 discrete voltage levels. The pulsegenerator accordingly has 32 outputs which are applied in turn to theterminals numbered 1-32 in FIGURE 6. A matrix composed of a series of 27resistors R R are connected to selected ones of the input terminals,through isolating diodes D D Only 26 resistors are used as not all ofthe discrete levels are required for the exemplary waveform. The lastresistor is connected in series with the load resistor R and thence tothe collector of an NPN transistor Trl. This transistor serves as a gateand there is accordingly applied to its base, from terminal 33, a lowspeed gating pulse having a waveform such as is shown in FIGURE 7a.

The output of transistor Trl, in parallel with the output of transistorsTr2 and Tr3, which are similar gating transistors of other resistancematrices, are applied to the base of a summing amplifier transistor T14,also of the NPN type. The collector lead resistor Rc of the summingamplifier is connected to a positive supply and feedback from thecollector to the base of the summing amplifier transistor is effected bymeans of a resistor Rfb in parallel with a by-pass capacitor Cfb. Thebase of the transistor is biased through resistor Rb.

This arrangement differs from that of FIGURES 1 and 2, in that theoutput is derivedas a function of the current through, rather than thevoltage across, the load resistance R L Consideration has been givenabove to the use of resistance matrices where the load resistance is inseries with the individual resistors of the matrix, and the voltage isderived from the voltage appearing across the common load resistor.Generally similar considerations apply to the case where, as in FIGURE6, the current through the load resistor is used as the significantparameter. Consequently, the same advantage obtains by the use of thesumming amplifier having heavy degenerative feedback. As shown, thetransistor amplifier, with the feedback ratio determined largely by theproportion of the resistors R and R can be arranged so as to maintain asubstantially constant potential at the base of the transistor.- Thefact that this is so is highly suitable when the effect of the additonalresistor matrices, gated through transistors Tr2 and Tr3, and any othersthat may be used, is considered.

We claim:

1. A waveform generator comprising a pulse source including a pluralityof outputs for generating a predetermined sequence of pulses having thesame amplitude, a resistor matrix comprising a plurality of resistorsjoined in series, an output device connected in series with the resistormatrix, a plurality of diodes connecting the outputs of said pulsesource each to a junction between adjacent resistors of said matrixwhereby each pulse of said sequence is applied across respectivepreselected resistors of said matrix and said output device in series;said output device comprises an amplifier device including an input andan output, a load resistor connected in series between the resistormatrix and the input of said amplifier, and negative feedback meansconnected between the output and input of the amplifier device wherebythe DC. gain of the amplifier device is substantially unity.

2. A waveform generator according to claim 1 further comprising anadditional fedeback means from the output to the input of the amplifierdevice which is capacitive to decrease its transient response.

3. A waveform generator comprising a plurality of pulse sources eachhaving a plurality of outputs, for generating a predetermined sequenceof pulses having the same amplitude, a plurality of resistor matriceseach comprising a plurality of resistors joined in series, an outputdevice connected in series with each resistor matrix, means connectingthe pulse source output to a junction between adjacent resistors of therespective matrix whereby each pulse of said sequence is applied acrossrespective preselected resistors of the respective matrix and saidoutput device in series; said output device comprises an amplifierdevice including an input and an output, a plurality of load resistorsand gate means connected in series between respective resistor matricesand the input of said amplifier device, control means for selectivelyoperating said gate means, and a negative feedback means connectedbetween the output and input of said amplifier device whereby the DC.gain of the amplifier device is substantially unity.

4. A waveform generator according to claim 3 further comprising afeedback capacitor in parallel with said negative feedback means tosmooth out the resulting waveform.

References Cited by the Examiner UNITED STATES PATENTS 2,472,774 4/1949Mayle 1785.1 2,857,462 10/1958 Lin 33O28 X 2,918,669 12/1959 Klein.2,963,579 12/1960 Berry 328-186 2,974,285 3/1961 Schenck 328101 X3,135,873 6/1964 Werme 30788.5

FOREIGN PATENTS 809,375 2/1959 Great Britain.

OTHER REFERENCES Karplus: Analog Simulation, chapter 9, 1958, pages 233and 249.

Electronic Fundamental and Application (Ryder), published byPrentice-Hall, September 1959, page 347 relied on.

JAMES D. KALLAM, Acting Primary Examiner.

JOHN W. HUCKERT, Examiner.

J. D. CRAIG, Assistant Examiner.

1. A WAVEFORM GENERATOR COMPRISING A PULSE SOURCE INCLUDING A PLURALITYOF OUTPUTS FOR GENERATING A PREDETERMINED SEQUENCE OF PULSES HAVING THESAME AMPLITUDE, A RESISTOR MATRIX COMPRISING A PLURALITY OF RESISTORSJOINED IN SERIES, AN INPUT DEVICE CONNECTED IN SERIES WITH THE RESISTORMATRIX, A PLURALITY OF DIODES CONNECTING THE OUTPUTS OF SAID PULSESOURCE EACH OF A JUNCTION BETWEEN ADJACENT RESISTORS OF SAID MATRIXWHEREBY EACH PULSE OF SAID SEQUENCE IS APPLIED ACROSS RESPECTIVEPRESELECTED RESISTORS OF SAID MATRIX AND SAID OUTPUT DEVICE IN SERIES;SAID OUTPUT DEVICE COMPRISES AN AMPLIFIER DEVICE INCLUDING AN INPUT ANDAN OUTPUT, A LOAD RESISTOR CONNECTED IN SERIES BETWEEN THE RESISTORMATRIX AND THE INPUT OF SAID AMPLIFIER, AND NEGATIVE FEEDBACK MEANSCONNECTED BETWEEN THE OUTPUT AND INPUT OF THE AMPLIFIER DEVICE WHEREBYTHE D.C. GAIN OF THE AMPLIFIER DEVICE IS SUBSTANTIALLY UNITY.